31-03-2014, 02:24 PM
The Data Link Layer
The Data Link.ppt (Size: 395 KB / Downloads: 50)
Data Link Layer : Introduction
broadcast links: many senders, potentially many receivers
framing
reliable communication
accessing a shared medium
addressing
many senders many receivers
HDLC: control field
control field format for "data" frames:
3-bit seq number
3-bit ack number
1 bit P/F to indicate sender-to-receiver to vice-versa
control field format for "supervisory" frames:
Broadcast links: Multiple Access Protocols
Single shared communication channel
two or more simultaneous transmissions by nodes: interference
only one node can send successfully at a time
question: how to share this broadcast channel examples of multiple access environments:
Taxonomy of MAPs (cont.)
Controlled access protocols: stations reserve or are assigned channel, no collisions
predetermined channel allocation: time division multiple access
demand adaptive channel allocation
reservation protocols
token passing (token bus, token ring)
CSMA/CD
CSMA with collision detection(CD):
listen while talking!
stop transmitting when another pkt has collided with your pkt
wait random time before attempting to resend
worst case time to detect a collision?
performance depends (as in CSMA) on channel length